Duplicate OSAtomic.h between aarch64 and x86_64

The reason this is required is for two reasons: 1) the libc
targeting `aarch64` macOS is slightly newer than that targeting
`x86_64`, and 2) `OSAtomic.h` uses relative imports rather than
system-wide imports for accompanying headers which clearly is an
oversight on Apple's part. Until such time when `libkern` headers
between `x86_64` and `aarch64` are identical, this will require a
manual intervention to duplicate the relevant headers between the
respective architectures.

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